B Question about Preventing Tidal Locking around Red Dwarf Stars


A couple of questions about planets around Red Dwarf Stars:

I'm imagining a planet around a Red Dwarf Star...think Trappist-1...and want to know how I can prevent tidal locking for an Earth-sized terrestrial planet in such a star's habitable zone....say, 1/50th to 1/20th of an AU from the star.

From what I gather, the close distance to the star would make it likely that any such planet would be tidally locked.

How to avoid this?

Would having a large moon allow this hypothetical planet maintain a regular rotation?

Or what if there was a large planet, maybe a mini-Neptune, next to it in orbit? Would that have any effect on the tidal-locking?



Science Advisor
The more evenly distributed the mass, the weaker the effect of tidal forces will be. Also, if the planet is far from the parent star, tidal forces will be weaker. Remember, the tidal forces are the result of the difference in gravitational pull across the planet. The distance from the center of the star to the near side of the planet vs. the distance from the center of the star to the far side of the planet. The more distant the planet is, the smaller the difference between the near and far side (comparatively).

Of course, in terms of habitability, this brings up a whole new set of problems.
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stefan r

Science Advisor
Gold Member
The moon rotating has angular momentum. The process that causes a planet to tidal lock will also cause a moon spiral in. Moons will escape in some cases because the hill sphere shrinks as a planet gets closer to the host star.

Suppose we think of a double planet as a single thing. The process of tidal locking will also be a process of making the double planet into a single planet. Next think of them as two objects orbiting each other. As they spiral in their rotational velocity increases and their orbital period decreases. The end result is a collision but it could be a long time before the collision occurs. Each of the individual planets will initially have their own rotational velocity. As they lock to each other that angular momentum gets converted to orbital momentum.
The planets tidal locking to each other will counter the pair locking with the star. So you can assemble the most extreme case. Two equal mass objects are positioned inside edge of the hill sphere and both are rotating near their breakup velocity. They eventually fall in to each other but that "eventually" could be a long time. Technically Earth would lock to the Sun except but will not happen within the Sun's lifetime. The habitable part is quite debatable. Planets near breakup velocity will lose atmospheres much faster than slow rotators. The closer you are to exact extreme conditions the less likely it will be that such a thing exists or that we will find one. It is possible but it is also very possible that life will adapt to conditions on a tidally locked planet.

Or what if there was a large planet, maybe a mini-Neptune, next to it in orbit? Would that have any effect on the tidal-locking?

Another planet orbiting the star would add additional tides. That will tend to dissipate rotational momentum. It makes it worse. You might have some sort of resonant orbit and rotation period.
Along those lines you can also have a planet in a binary star system. You can tidal lock to the red dwarf and get enough heat flux to prevent a permanent ice sheet. The second star can illuminate the antipode enough to enable plant life. Jupiter gets more sunlight than the floor of some dense forests on Earth. You can get seasons with elliptical orbits.
The planet could also be in orbit around the k-dwarf (or brown dwarf or white dwarf) and have the system orbit near the habitable zone of a larger star. That gives you every down side of being near the k-dwarf without adding much upside for the evolution of life that I am aware of. It slightly increases the total volume of habitable zone available in the Milky Way. It would be a great setting for sci-fi and a good target for colonization.

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